The src-family of tyrosine kinases are ubiquitously expressed proteins that have been implicated in the regulation of growth, differentiation and transformation. As these proteins are kinases, understanding their mechanism of action will necessitate the identification of the proteins that are their substrates. Recently, we and others, identified a protein that binds to and is phosphorylated by src-family tyrosine kinases. This protein, p62, was first noted as a major tyrosine phosphorylated protein in transformed and growth factor treated cells. Although its function is unknown, it is known to bind ras-GAP and has therefore been implicated in the regulation of ras. The cloning of p62 in 1992 showed that p62 was related to RNA binding proteins, that it had a tyrosine-rich C-terminus and that it contains at least 5 proline-rich motifs. As phosphorylated p62 binds src-family kinases, phospholipase C and grb2 as well as ras-GAP via both SH2 and SH3 mediated interactions, we have proposed that p62 serves as a scaffolding for the recruitment of SH2 and SH3 containing signalling proteins in src- mediated signalling pathways. We propose to test this model as well as try and understand the role of the RNA binding domain of p62. Recently, a 68kD protein was described that is closely related to p62. As we have reagents that distinguish between the two, our first goal is to determine the relationship between p62 and p68. Our second goal, is to determine the function of p62/p68 in signal transduction pathways. Experiments are proposed to study the role of p62 in transformation as well as cell cycle regulation. Lastly, we propose to study the structural features of the p62 RNA binding domain and its regulation by tyrosine phosphorylation. We will then attempt to identify the cognate RNAs which bind to p62/p68 in vivo. As the phosphorylation of p62 has been demonstrated to be an almost ubiquitous feature of transformation and growth factor treatment, the findings generated in these studies will have immediate relevance to our understanding and possibly the treatment of human cancers.